503 Idle Issues!!!

[chriswebb]

Good morning Guys and Girls,

I was wondering if anyone might be able to help with my 503 DCDI issue?

Ive only recently had the aircraft (Rans S6 Taildragger) and so not sure how long this problem may have ben going on.

the issue i have is the idle speed sticks to 3000rpm once the engine has been used for flight. during the winter i ran the engine and got it up to temp whilst practicing my taxiing, when i throttled back to idle it always dropped to 1800-2000rpm and started to 'shake' which i have been told is about right.

However given the recent good weather i have been flying and each time i have landed with the throttle closed the engine will idle at 3000rpm. I have adjusted the idle adjuster on the carbs (knurled thumb screw) and this does drop the idle speed at the time but when i went up next the same problem can back!

with the engine off i have fully opened and closed the throttle and you can hear the slide (not sure what its called) hitting the stops and there is slack on the cable when the throttle is fully closed.

there is the provision for a choke but no cables fitted as i use a primer, is it possible that the engine is drawing fuel through the primer? (new primer installed Nov 2014)

If anyone has any ideas of where to look it would be greatly appreciated.

Thanks

Chris

[JohnLindsay]

Hi Chris,

I don't profess to know anything about the 503 (or much else for that matter!) but I wonder if you have an air leak somewhere downstream of the carburettors, i.e. associated with the inlet manifolding and the various joints and gaskets. It might be a crack or a loose joint that opens up when the engine is truly hot after flight. Taxiing during the winter will have got the engine up to temperature on the gauge but I doubt that the heat will have properly soaked into all the peripherals such as the carbs and manifolds. A small air leak is all you would need to raise the idle speed. Just a thought.

Good luck tracking the problem down.

John.

[mikehallam]

More to check thoughts.

a) If the cable is too slack it could hook up on the edge of the cable abutment socket when you think it's closed.
b) Fuel primer in leak really means more fuel, not more air, so ought to slow it down.
c) Cables routing to Carbs. has to be in smooth curves; could have been not optimum and/or moved a little away from best line.
d) Bing primer is not a true choke, but an auxiliary enrichment device with an air feed and its own jet
It is operated by a separate cable with lifts a plunger off a 'lavatory seat' on one side in each carb. So I assume that in removal of the 'choke', that plunger(s) stay there still and sprung down onto its seat (s) ?

mike hallam.

[chriswebb]

Thanks for the responses,

I have had a look over the weekend and
the cable slack is fine as there is not enough to get caught on anything,
I have checked the carb rubbers and they are good,
I have brought and awaiting delivery of a new set of gaskets, so i will replace them at the earliest opportunity,
I have checked the choke plunger and it has a spring return holding it down holding it to its seat on both carbs so they are good,
the cable routing are smooth and secure,
I have looked at the primer system and it appears that the fuel is pumped directly to the manifold side of the carbs bypassing them completely, but as you said Mike more fuel is not the issue,
hopefully it is just the inlet gaskets as i cant see any cracks on the manifolds but failing that i may be searching for some spares.

Other than Afors any ideas on where to look, I dont particularly wish to be ripped off by this countries aviation industry but i am looking for genuine parts.

Cheers

Chris

[mikehallam]

Looks like you've done all the right things - except find out why !

My experience BTW was with the single carb. 447 two-stroke Rotax.

However on that particular set up the Ali manifold split feed to the two cylinder inlet ports and the one carb. connected to it & and was supported via a rubber 'tube' secured by two Jubilee clips.
These rubbers could I suppose crack & yet be hard to see whilst in situ. So if running out of ideas, remove them and inspect closely. Personally I never had a problem with the manifold to cylinder joint and I assume the studs are secure ?
When you hear the slides shut to the stops are you really sure ? Can you visually confirm that both sides are right down by removing the air filter and looking at the slides ?
The carb's. can be tilted (rotated) in their rubber connections, they should be horizontal when the engine is ditto. i.e. carbs. in line with the side of the block & not with the ground when the 'plane is at rest. [In a tail wheeler as I had that was important].

If/when you can please send me some pic's and/or the solution for publishing in my free e-mail news sheet, "RansMail".

mike hallam. [email protected]

[chriswebb]

Hi Mike

I had dismissed the carb rubbers as i had assumed that as the issue only manifested itself after getting hot that this would rule them out but i certainly have a damn good look at them.
I have only assumed that the carbs slide closed by listening but i will take the filter off and visually check the slides are fully closed, and as my Rans is a taildragger I will check that they are correctly aligned with the engine and not the ground.

I'm hoping to get to the airfield this weekend and solve these issues, I will take some photos of my findings and email them to you.

Thanks

Chirs

[mikehallam]

Only one other thought before you get too hung up on the "problem".

When ticking over hot after flight on the ground it did run faster at the same low throttle position.
In my 477 experience it is a lot slower as slow as when started from cold and warming up, at this stage more throttle is needed to keep the rev's up & avoid gearbox chattering.

When landing,say on 'finals', with throttle fully closed you don't want the engine to stop, nor of course to rev. too much and make the a/c float. However at the reduction ratio Rotax employ, the propeller is at engine 3,000 rpm only doing about 1200 rpm which isn't too bad.

mike.

[chriswebb]

Cheers Mike,

The issue is that the additional revs are causing the a/c to float.

So over the weekend i had a look at the intakes and initially all looked good. but whilst i was there i noticed a leak from one of the spark plugs, turns out it was cross-threaded so i have removed the head and taken in to get an insert put in.
so with the head off i took the opportunity to have a closer look and although the intakes were secure they is signs that it wasn't sealed fully.

I hopefully get the head back tomorrow and will be putting her back together on the weekend.

There is some carbon build up, and as i have the engine in pieces i am going to clean it up, any advice on what i can use to remove the carbon?

[mikehallam]

Thanks for the direct mailed pictures.

Pity about the plug, but IMHO the small weep there and the witness on the manifold connection don't really look bad. Naturally the plug must be in properly.

Rans 912 book specifies Silicon paste rather than Copper Slip on its threads. So I changed to that too for the two-stroke plugs in the 477. Meant better heat conductivity plug across the r=threads to the cylinder head and when removed annually (say 20 to 30 hours)
intervals they came out easily and cleanly. The paste doesn't deteriorate much in service.

Could be enough engine suck at closed throttle and on finals for some weakening via the gasket of entering fuel/air/oil mix. Again it obviously must be attended to and perhaps with good fortune you'll find it's better afterwards.

Otherwise I'm out of ideas. As for carbon, I'm not a good adviser - all I used was a piece of soft Ali rather than any proprietary softening/cleaning liquids etc.

Pleas write again as if nothing else RansMail will be happy to publicise this mid May is the next one.

mike hallam.

[mikehallam]

P.S.

There are more 503 engine users on the [b]BMAA [/b]forum also perhaps the [b]'Microlight forum' [/b].
Cross posting the problem there might well bring more feed-back.
If you need me to help start that off, as I'm registered on both (though they're both free) pls. mail me.

mike hallam.

[chriswebb]

I'm hopefully heading to the airfield the weekend.

Now I've cleaned the the carbon off the heads and cylinders (i thought I might as well as I had it in pieces and it looked like it needed doing)

Do i need to run the engine in?

I will run the engine to make sure all is good but do i have to do a full run in?

[mikehallam]

Not required after a 'de-coke' as the pistons & cylinder walls all remain the same.

Provided you didn't (unlikely from what you write) remove pistons & failed to put them back to their original orientation and cylinder.
In general terms Aero engines aren't really 'run-in' as the rings must work hard to make the correct sort of long term surface.
The old style slower, lighter operation is said to promote glazing & poorer sealing. This is 4-stroke guidance, but an already in service & well used reassembled two-stroke may be run as hard as usual.

mike.